This invention relates to the field of wireless communication systems and more particularly to controlling radiated RF power level during establishment of a call and on an ongoing basis in a cellular wireless system, such control of power using an estimate of the quality of a received angle-modulated carrier.
In a wireless communication system, as a general rule, it is highly desirable that the minimum radiated radio frequency (RF) carrier power necessary to achieve a specified quality level of communications be used in order to conserve energy and, perhaps more importantly, in order to reduce interference with other users of a shared RF spectrum. With the increasing use of cellular wireless communication systems comprising a base station (BS) at each cell, and remote terminals (a remote terminal also is called a subscriber unit (SU) or a subscriber station) communicating with an assigned base station, the problem of interference between stations within a given cellular area, and between neighboring cells, requires intelligent interference management in order to more effectively use the allocated common RF bandwidth. Such interference management is the goal of power control. As a general rule, the minimum radiated RF power required for maintaining an acceptable quality of service should be used.
Two types of power control are necessary: initial power control, and ongoing power control. In initial power control, the goal is to initiate communications with the minimal level of power necessary to achieve an acceptable level of communications. Ongoing power control maintains minimum transmitted power usage on a link as the communication system changes over time by new links being formed while others are being established.
Initial Power Control
Several communications protocols are known for cellular systems, including, for example, the Personal Handiphone System (PHS) and the Global System for Mobile communications (GSM). Both use time division multiple access (TDMA) together with frequency division multiple access (FDMA) techniques. Such communications protocols all include protocols for call establishment, for example for a subscriber unit initiating communications to a BS, or a BS initiating communications with a SU. Some of these protocols may not include initial power control. There thus is a need in the art for an initial power control method that may be applied to an existing communication system without adversely impacting communication system protocols that are in existence.
Ongoing Power Control
Ongoing power control is the control of radiated power as the communication environment changes after initial communications is achieved. For example, when the radiated power is increased in a particular link between a SU and a BS in order to achieve an acceptable quality for the received signal, or for some other reason, such a change may cause unacceptable quality changes for other stations using either the same or adjacent channels. In addition, as new connections are established and on-going connections are disconnected, power assignments might change resulting in changes (for better or worse) in the quality of existing connections. For example, xe2x80x9cexcess qualityxe2x80x9d may result, implying that excess RF power is being used under the new conditions. Degraded quality also may be experienced, implying that some connections may require greater radiated RF power. Variations in propagation characteristics, atmospherics, and man-made interference can also cause changes that require adjusting RF power levels. This is the goal of ongoing power control.
Spatial division multiple access (SDMA) techniques are known in which the same xe2x80x9cconventional channelxe2x80x9d (i.e., the same frequency channel in a frequency division multiple access (FDMA) system, timeslot in a time division multiple access (TDMA) system, code in a code division multiple access (CDMA) system, or timeslot and frequency in a TDMA/FDMA system) may be assigned to more than one subscriber station. This is done by using an antenna array of several antenna elements at the base station, and on the uplink (communications from a subscriber unit to a base station), the signal from each antenna element is weighted in amplitude and phase by a receive weight (also called spatial demultiplexing weight), all the receive weights determining a complex valued receive weight vector which is dependent on the receive spatial signature of the remote user. The receive spatial signature (also called the receive manifold vector) characterizes how the base station array receives signals from a particular subscriber unit. On the downlink (communications from the base station unit to a subscriber unit), transmission is achieved by weighting the signal to be transmitted by each array element in amplitude and phase by a set of respective transmit weights (also called spatial multiplexing weights), all the transmit weights for a particular user determining a complex-valued transmit weight vector which also is dependent on the spatial signature of the remote user. When transmitting to several remote users on the same conventional channel, the sum of weighted signals is transmitted at the antenna arrays.
The weighting of the signals either on the uplink from each antenna element in an array of antennas, or on the downlink to each antenna element is called spatial processing herein. Spatial processing is useful even when no more than one subscriber unit is assigned to any conventional channel. Thus, the term SDMA shall be used herein to include both the true spatial multiplexing case of having more than one user per conventional channel, and the use of spatial processing with only one user per conventional channel to mitigate adjacent channel interference and adjacent cell interference, reduce the cellular frequency reuse factor, etc. The term channel shall refer to a communications link between a base station and a single remote user, so that the term SDMA covers both a single channel per conventional channel, and more than one channel per conventional channel.
Methods for determining spatial receive and transmit weight vectors are known in the art. See for example, U.S. Pat. No. 5,515,378 (issued May 7, 1996) and U.S. Pat. No. 5,642,353 (issued Jun. 24, 1997) entitled SPATIAL DIVISION MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEMS, Roy, III, et al., inventors; U.S. Pat. No. 5,592,490 (issued Jan. 7, 1997) entitled SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS, Barratt, et al., inventors; U.S. patent application Ser. No. 08/735,520 (filed Oct. 10, 1996), entitled SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING, Ottersten, et al., inventors; U.S. patent application Ser. No. 08/729,390 (filed Oct. 11, 1996) entitled METHOD AND APPARATUS FOR DECISION DIRECTED DEMODULATION USING ANTENNA ARRAYS AND SPATIAL PROCESSING, Barratt, et al., inventors (hereinafter xe2x80x9cOur Demodulation Patentxe2x80x9d); and U.S. patent application Ser. No. 08/948,772 (filed Oct. 10, 1997) entitled METHOD AND APPARATUS FOR CALIBRATING A WIRELESS COMMUNICATION STATION HAVING AN ANTENNA ARRAY, Parish, et al., inventors (hereinafter xe2x80x9cOur Calibration Patentxe2x80x9d), each of these incorporated herein by reference in their entirety, these patents or applications collectively referred to herein as xe2x80x9cOur Spatial Processing Patentsxe2x80x9d. For example, in systems that use time division duplexing (TDD) so that uplink and downlink communications occurs over the same frequency (in a FDMA or a TDMA/FDMA system), a receive weight vector of receive weights determined on the uplink can be used to determine the required transmit weight vector of transmit weights for communications on the downlink from the base station to the same remote subscriber unit.
No practical methods of ongoing power control are known in the prior art that are applicable to systems using SDMA techniques, in that the power control methods can effectively adjust all of the SDMA system parameters required for minimizing the total radiated RF power while maintaining acceptable quality levels for all channels. Using SDMA introduces substantial complexities in the RF radiated power control problem because determining weight vectors affects power control, and vice versa. Any change in RF power on a conventional channel using SDMA will affect the transmit and receive weight vectors assigned to users using the same conventional channel and any change in the spatial processing effects the power required by existing users in order to maintain an adequate communication quality level. Prior art methods for power control typically do not account for the specific aspects of SDMA, and may cause instability in such an SDMA system, wherein an improper choice of transmit power adversely alters the spatial multiplexing (i.e., transmit) and demultiplexing (i.e., receive) weight vectors, causing the transmit powers to deviate further from optimum until signal quality and network capacity are both degraded.
The optimal solution of the ongoing power control problem for an SDMA system requires the simultaneous solution of the SDMA multiplexing weight assignment problem and the power assignment problem. This at the very least is an involved computational task, and to date has been an intractable and overwhelming computational task. Thus there is a need in the art for a practical near optimal method for determining spatial processing weight vectors and ongoing power control for an SDMA system.
The objective of ongoing power control problems for communications is to minimize the total power transmitted in the communication system while ensuring that a desired (xe2x80x9ctargetxe2x80x9d) signal to interference-plus-noise ratio (SINR) for every connection within every cell is achieved. When expressed in this way, the resulting power control method is referred to as a globally optimal method. Such a globally optimal method in general requires communications between base stations of the system. Locally optimal methods are those for which optimality is satisfied within some subset of the overall system, for example, within a particular cell. There may be practical difficulties with directly determining a globally optimal method when dealing with a large number of intercell and intracell connections. For example, the computation time may be too long relative to the rate of change of connection conditions; and, it may not be feasible or practical to gather the necessary data, such as the path gain between every base station and every remote subscriber unit in real time. It has been shown (Yun, L. C. M., Transport for Multimedia on Wireless Networks, Doctoral Dissertation, University of California, Berkeley, Calif., 1995) for a non-SDMA system that, by incorporating the effects of interference coupling between cells, the localized control strategy can be made to asymptotically converge to the globally optimal solution. Thus there are advantages to having an ongoing power control strategy that uses locally optimal power control. Thus there is a need in the art for locally optimal power control methods for systems with SDMA that are xe2x80x9cdistributed,xe2x80x9d in that no inter-base station communication of power control information is required for operation.
Signal Quality Estimation
In order to implement power control, an objective measure of the quality of the received signal is required. It is generally accepted that a measure of the error in the signal is a useful objective measure of quality. It is desirable that any such measurement of error be made while normal communications are taking place.
Several prior art methods exist for estimating the quality of received signal. One class or prior art techniques uses a measure of the received signal power as a measure of the received signal quality. An example is the commonly used received signal strength indicator (RSSI). The problem with such measures is that they do not distinguish between the desired signal and any interfering signals and/or noise. To overcome this shortcoming, some prior art power control methods use a measure of the bit error rate (BER) or the easier to obtain frame error rate (FER). For example, the initial power control method used in the IS-95 CDMA standard uses FER. FER is easier to obtain in practice than the BER because cyclic redundancy check (CRC) bits usually are part of a frame structure. The FER may be viewed as an approximate indication of the BER. Two main shortcomings of BER and FER as measures include:
1. It takes a long time (many frames) to accumulate a statistically meaningful estimate of BER or FER, which may be too slow for power control; and
2. The BER (or FER) may not be only a function of power, but may also be affected by other causes of a demodulation error. For example, residual frequency offset (even after any frequency offset correction has been applied) may contribute to the modulation error.
Additionally, prior-art decision-directed modulation error estimation methods exist which have used for quality estimation an error vector that represents the difference between the received signal and an idealized model of the signal that should have been received. The idealized model is generated from the detected bits by passing the detected bits through a bits-to-symbol mapper which converts the bits to the correct symbols, and then passing the correct symbols through a pulse shaper to produce the idealized model of the signal (a reference signal). The pulse shaper also needs to undo frequency correction and undo timing alignment. The difference between the resulting idealized model of the modulated signal (with any frequency offset and timing misalignment) and the actual received signal is used to estimate the noise and interference present in the actual signal, and this is used as a quality estimate.
This prior-art decision based quality estimator has several undesirable properties, some similar to the BER and FER measures:
1. a demodulation error may cause a large error in the quality estimate by substituting an incorrect symbol in place of the actual symbol that was meant to be transmitted;
2. frequency offset contributes to the modulation error;
3. measurement of modulation error does not directly relate to the RF carrier strength and to the noise and interference levels; and
4. estimation of the signal to interference-and-noise ratio (SINR) from the modulation error tends to result in a high variance (unreliable) estimator.
Note that the sensitivity to frequency offset is particularly undesirable when the quality estimator is for transmitter power control. Increasing the transmitter power because a frequency offset error is mistaken for noise or interference error, is not only completely ineffective, but is undesirable because an unnecessary excess transmitter power will cause increased interference with other system users.
Thus there is a need in the art for power control methods that use a process for estimating the quality of received signal which (a) is fast; (b) is substantially insensitive to frequency offset variations; and (c) leads to a measure, for example the signal to interference-and-noise ratio (SINR), that differentiates signal from interference and noise.
Thus one object of the present invention is a method and apparatus for ongoing power control in a system that includes SDMA. Another object of the invention is a method and apparatus for estimating received signal quality (as expressed by the signal to interference and noise level (SINR)) for use in the power control method and for other applications. Another aspect of the invention is an initial power control method and application using the signal quality estimation method and/or apparatus. Yet another aspect of the invention is a method for combined initial and ongoing power control applicable to a system that includes SDMA.
In one aspect of the present invention, a method for ongoing uplink power control for communications from one or more remote users to a communication station with SDMA is disclosed that includes separating the joint determination of a spatial weight vector of weights for receiving from a particular remote user and ongoing power control from that user""s transmitting into a receive weight vector determining part and a separate transmit power adjustment part. The method starts with one part, for example power adjustment wherein an initial power control strategy is used for transmitting from the remote user. Transmit power according to this initial strategy is assigned and transmission carried out. Receive weight vector assignment is now carried out for the signals transmitted to the communication station with this assigned transmit power. The resulting new weight vector is used and may affect the quality of communication. An estimate of the quality of communication is obtained for communication using the newly determined receive weight vector. Ongoing power control adjustment is applied using the estimate of the quality of communication, leading to a new power assignment for transmitting from the remote user. These new power assignments are used leading to new receive weight vector determination. Thus iterating between the transmit power setting and the spatial processing receive weight vector determining parts, receive weight vectors and power control are jointly determined.
In another aspect of the present invention, a method for ongoing downlink power control for communications to one or more remote users from a communication station with SDMA is disclosed that includes separating the determination of a complete transmit weight vector of weights for transmitting from the communication station to a particular remote user, the complete transmit weight vector comprising a set of relative transmit weights and a scaling to apply to the weights, into a relative transmit weight vector determining part and a separate transmit power adjustment part. The method starts with one part, for example power adjustment wherein an initial power control strategy is used for transmitting from the communication station to the remote user using some initial relative transmit weight vector of initial relative transmit weights. Transmit power according to this initial strategy is assigned and transmission carried out. An estimate of the quality of communication is obtained for communication using the initial transmit weight vector. Based on this, ongoing power control adjustment is applied using the estimate of the quality of communication, leading to a new power assignment for transmitting from the communication station, leading to new receive weight vector determination. Separately, an updated relative transmit weight vector is determined, and such an updated relative transmit weight vector is used for transmitting. Thus the complete transmit weight vector (which includes the set of relative transmit weights and the power setting) is obtained by separately determining the power setting and the spatial processing relative transmit weight vector.
Another aspect of the invention is a method for ongoing power control for uplink communications between one or more remote transmitters (e.g., SUs) and a receiving communication station (e.g., a BS), the communication station including an array of receiving antenna elements and spatial receive processing according to a set of receive weights (a weight vector). The method includes for communicating with a particular remote transmitter setting up initial power assignment for the particular transmitter, preferably according to the method described in the Parent Patent. Starting with the initial power assignment, a set of uplink weights (i.e., a receive weight vector) is determined at the communication station for the particular remote transmitter. This weight vector is used to determine a signal from the particular remote transmitter, the determining from a plurality of signals received at the antenna elements. The received signal quality for the signals from the remote transmitters at the communication station are estimated, and based on the received signal quality estimates, new power assignments are determined for the remote transmitters. The received signal quality preferably is an estimate of the SINR. The new power assignment is applied at the remote transmitters. Preferably, the power assignment is determined at the communication station and the remote transmitters are commanded to change power by the communication station. The remote transmitters transmit with these new uplink powers, and the processes of uplink weight vector determination and power control are now repeated. Preferably, power determination is carried out at prescribed intervals.
Another aspect of the invention is a method for ongoing power control for downlink communications between a transmitting communication station (e.g., a BS), and one or more remote receivers (e.g., a SU), the communication station including an array of transmitting antenna elements and spatial transmit processing to a particular remote receiver according to a set of transmit weights (i.e., a transmit weight vector). The method includes setting up initial power assignments for the communication station, preferably according to the method described in the Parent Patent, and transmitting from the communication station using the initial power assignment and initial transmit weight vector, one weight vector for each remote receiver. In the preferred embodiment, the communication station includes a set of receive apparatuses each apparatus coupled to one of the antenna elements of the array for receiving signals, and a receive processor for spatially processing the signals received at the antenna elements from any remote transmitter according to a receive weight vector. In the preferred embodiment, each remote receiver also includes a remote transmitter for transmitting a signal, and the transmit weight vector for transmitting from the communication station to a particular remote receiver are determined from signals received at the communication station antenna array as a result of transmitting of a signal by the remote transmitter at the particular remote receiver, and in particular, the transmit weight vector is determined from the receive weight vector determined for receiving the signal transmitted by the remote transmitter at the particular remote receiver to the communication station. An estimate is determined of the quality of the signals received at each remote receiver. Preferably, the signal quality estimates are each an estimate of the SINR at each remote receiver. Preferably, each remote receiver performs the SINR estimation and reports the received signal quality to the communication station at prescribed intervals. Based on the signal quality estimates, downlink power assignments are determined for communicating with each remote receiver. The new assignments are used by the communication station to transmit to the remote receivers. The quality estimation, power assignment and transmission are then repeated. In the preferred embodiment, the same weight vector as previously used is used if no weight vector updating has occurred, and an updated transmit weight vector is used if an updated weight vector is available.
Another aspect of the invention is a method for global ongoing power control for a communication system, which includes a set of one or more communication stations. In the system, each communication station communicates on the uplink with a set of one or more corresponding remote transmitters and on the downlink with a set of one or more corresponding remote receivers co-located with the corresponding remote transmitters. Preferably, the system is a cellular system, each communication station is a base station, and each remote transmitter and co-located remote receiver is a subscriber unit communicating with its corresponding base station. Each communication station including an array of receiving antenna elements, a set of receive apparatuses couple to the antenna array, with the outputs of the receive apparatuses coupled to a receive spatial processor, communication with a particular corresponding remote transmitter being according to a set of receive weights (i.e., a weight vector). Each communication station also includes an array of transmitting antenna elements, a set of transmit apparatuses coupled to the antenna elements and a transmit spatial processor forming a set of signals for the transmit apparatuses, communication with a particular corresponding remote receiver being according to a transmit weight vector. Power control for the overall system includes using the above method for ongoing power control for uplink communications and the above method for ongoing power control for downlink communications. The power assignment steps in both the downlink and the uplink ongoing power control methods include jointly determining all the sets of transmit powers that minimize a weighted sum of all the transmit powers for communications between the sets of corresponding remote transmitters and the communication stations (for uplink ongoing power control) and between the communication stations and the sets of corresponding remote receivers (for downlink ongoing power control) under the constraint of maintaining an acceptable level of communication for each communication link between any transmitter (in a communication station on the downlink and in a remote transmitter on the uplink) and any corresponding receiver (in a corresponding remote receiver on the downlink and in a communication station on the uplink). In one embodiment, the weighted sum of transmit powers is the sum of all transmit powers, the acceptable level of communications is a target SINR, and the target SINR is the same for all uplink communications and is the same for all downlink communications.
In another embodiment of the method for global ongoing power control, the power assignment step in the ongoing power control for uplink communications is carried out independently at each communication station and that communication station""s set of corresponding remote transmitters, and the power assignment step in the ongoing power control for downlink communications is carried out independently at each communication station and that communication station""s set of corresponding remote receivers.
One embodiment of the power assignment step in the ongoing power control method for uplink communications includes periodically updating the power transmitted from a remote transmitter to the communication station as a function of the target signal quality for communicating to the communication station, the powers used in previous updates for transmitting from the remote transmitter, and the estimates of the previous quality of the signal received at the communications station from the remote transmitter. The update period in the preferred embodiment is two frames. Preferably, the signal quality estimate is an SINR estimate and the target signal quality is a target SINR. In one version of the power assignment step, the function is only of the target SINR, the most recent SINR estimate, and the most recently applied power. When all power and SINR quantities are in logarithmic scale, in a particular version, the difference between the power to apply in the next update and the power applied in the most recent update is some function of the difference between the most recent SINR estimate and the target SINR, and preferably this function is proportionality. In the preferred embodiment, the target SINR is the same for all spatial channels on a conventional channel.
One embodiment of the power assignment step in the ongoing power control method for downlink communications includes periodically updating the power transmitted from the communications station to a remote receiver as a function of the target signal quality for communicating to the remote receiver, the powers used in previous updates for transmitting from the communication station to the remote receiver, and the estimates of the previous quality of the signal received at the remote receiver from the communications station. The update period in the preferred embodiment is two frames. Preferably, the signal quality estimate is an SINR estimate and the target signal quality is a target SINR. In one version of the power assignment step, the function is only of the target SINR, the most recent SINR estimate, and the most recently applied power. When all power and SINR quantities are in logarithmic scale, in a particular version, the difference between the power to apply in the next update and the power applied in the most recent update is some function of the difference between the most recent SINR estimate and the target SINR, and preferably this function is proportionality. in the preferred embodiment, the target SINR is the same for all spatial channels or a conventional channel.
In another embodiment of the uplink power assignment step, on a conventional uplink channel, the set of powers to apply for uplink communications for the spatial channels on the conventional uplink channel are these that minimize the weighted sum of the powers to transmit on the uplink spatial channels of the conventional uplink channel from the remote users to the communication station, constrained by the requirement of maintaining an acceptable (i.e., target) quality of communication on any particular uplink spatial channel of the conventional uplink channel. In a particular implementation, the minimization criterion is to minimize the total of the powers to transmit, and the constraint is that a predicted uplink signal quality measure, preferably the predicted SINR on any particular uplink spatial channel is at least some target SINR for that particular uplink spatial channel, where the predicted uplink SINR for a particular spatial channel is an expression of the particular spatial receive weight vector for the particular uplink spatial channel, the uplink path losses for the particular uplink spatial channel and for other uplink spatial channels of the conventional uplink channel, the receive spatial signature of the remote transmitter on the particular uplink spatial channel, the receive spatial signatures of the other remote transmitters on the conventional uplink channel, and the post-spatial processing noise-plus-intercell interference experienced by the communication station on the particular uplink spatial channel. In a particular embodiment, the path loss for any spatial channel is a function of the estimated SINR and of the most recently used transmit power. The intercell interference plus noise for any uplink spatial channel is determined as a function of the SINR estimate for that uplink spatial channel, the receive weight vector and the receive spatial signatures for all uplink spatial channels on the conventional uplink channel, the powers by the remote transmitters applied in the previous update of the uplink power control method for communicating on all the uplink spatial channels of the conventional uplink channel, and the path losses for the particular uplink spatial channel and for the other uplink spatial channels on the conventional uplink channel. In a particular implementation, the particular constraint for the particular uplink spatial channel, denoted by subscript i, of a total number (denoted by d) of spatial channels on a conventional channel that a predicted uplink signal quality measure, preferably the predicted SINR, is at least the value of a target signal quality, preferable a target SINR for the particular uplink spatial channel (denoted by SINRtargetiU) can be mathematically expressed as                     L        i        U            ⁢                        "LeftBracketingBar"                                    w              i                              U                *                                      ⁢                          a              i              U                                "RightBracketingBar"                2            ⁢              p        i        U                                      ∑                                    j              ≠              i                        ,                          j              =              1                                d                ⁢                              L            j            U                    ⁢                                    "LeftBracketingBar"                                                w                  i                                      U                    *                                                  ⁢                                  a                  j                                            "RightBracketingBar"                        2                    ⁢                      p            j            U                              +              I        i        U              ≥      SINR          target      i        U  
where, for j=1, . . . , d, pjU is the power for transmitting on uplink spatial channel j from the transmitting remote user to the communication station on uplink spatial channel j, LjU is the path loss (which might be a gain if larger than 1) on uplink spatial channel j from the transmitting remote user to the communication on uplink spatial channel j, wjU is the uplink (i.e. receive) weight vector (of weights) for receiving from the user on uplink spatial channel j, with the receive weight vector having a Euclidean norm of 1, ajU is the transmit spatial signature of the remote user on uplink spatial channel j, the uplink spatial signatures each having Euclidean norm 1, and IjU is the uplink post-spatial processing noise-plus-intercell (i.e., out-of cell) interference experienced by the communication station on uplink spatial channel j. In a particular implementation, the target SINRs are the same for all uplink spatial channels of the conventional uplink channel. The uplink minimization problem in general is to find the set of positive piU such that       ∑          i      =      1        d    ⁢            c      i      U        ⁢          p      i      U      
is minimized subject to the constraint of the predicted signal quality being at least the target signal quality being met on every uplink spatial channel of the conventional channel.
In yet another embodiment of embodiment of the uplink power assignment step, in a conventional uplink channel, the set of powers to apply for uplink communications for the spatial channels on any conventional uplink channel are determined by setting the predicted uplink SINR in each uplink spatial channel of the conventional uplink channel to be equal to a target SINR for that uplink spatial channel. In the preferred embodiment, the target SINR is the same for all uplink spatial channels of the conventional uplink channel.
In another embodiment of the downlink power assignment step, in a conventional downlink channel, the set of powers to apply for downlink communications for the spatial channels in the conventional downlink channel are those that minimize the weighted sum of the powers to transmit on the downlink spatial channels of the conventional downlink channel from the communication station to the remote receivers on the conventional channel, constrained by the requirement of maintaining an acceptable (target) quality of communication in any particular downlink spatial channel of the conventional downlink channel. In a particular implementation, the minimization criterion is to minimize the total of the powers to transmit on the conventional downlink channel and the constraint is that a predicted downlink signal quality measure (the measure preferable the predicted downlink SFNR) for the remote receiver on any particular downlink spatial channel is at least some target signal quality, preferably a target SINR for the particular downlink spatial channel, where the predicted downlink SINR for the particular spatial channel is an expression of the particular spatial transmit weight vector in the particular downlink spatial channel, the other transmit weight vectors used for communication in the other downlink spatial channels of the conventional downlink channel, the downlink path losses for the particular downlink spatial channel and for other downlink spatial channels of the conventional downlink channel, the transmit spatial signature for transmitting to the remote receiver on the particular downlink spatial channel, and the post-spatial processing noise-plus-intercell interference experienced by the remote receiver on the particular downlink spatial channel. In the particular embodiment, the path loss for any spatial channel is a function of the estimated SINR at the remote receiver and of the most recently used transmit power. The intercell interference plus noise for any spatial channel is determined as a function of the SINR estimate for the remote receiver on the particular spatial channel, the transmit weight vectors and the transmit spatial signatures for all downlink spatial channels in the conventional downlink channel, the powers applied in the previous update of the power control method for communicating in all the downlink spatial channels of the conventional downlink channel, and the path losses for the particular downlink spatial channel and for the other downlink spatial channels in the conventional downlink channel. In a particular implementation, the particular constraint for the particular downlink spatial channel, denoted by subscript i, of a total number (denoted by d) of downlink spatial channels in a conventional downlink channel that the predicted SINR at the remote receiver on the particular downlink spatial channel is at least the value of the target SINR for the particular downlink spatial channel (denoted by SINRtargetiD can be mathematically expressed as                     L        i        D            ⁢                        "LeftBracketingBar"                                    w              i                              D                *                                      ⁢                          a              i              D                                "RightBracketingBar"                2            ⁢              p        i        D                                      ∑                                    j              ≠              i                        ,                          j              =              1                                d                ⁢                              L            i            D                    ⁢                                    "LeftBracketingBar"                                                w                  j                                      D                    *                                                  ⁢                                  a                  i                                            "RightBracketingBar"                        2                    ⁢                      p            j            D                              +              I        i        D              ≥      SINR          target      i        D  
where, for j=1, . . . , d, pjD is the power for transmitting in downlink spatial channel j from the transmitting communication station to its remote receiver on downlink spatial channel j, LjD is the path loss (which might be a gain if larger than 1) in downlink spatial channel j from the transmitting communication station to the remote receiver on downlink spatial channel j. wjD is the downlink (i.e. transmit) weight vector (of weights) for transmitting to the user on downlink spatial channel j, with the vectors each having Euclidean norm 1, ajD is the transmit spatial signature of the remote user on downlink spatial channel j, the downlink spatial signature having a Euclidean norm of 1, and IiD is the downlink post-spatial processing noise-plus-intercell (i.e., out-of cell) interference experienced by the receiver on the particular downlink spatial channel i. The downlink minimization problem in general is to find the positive set of piD such that       ∑          i      =      1        d    ⁢            c      i      D        ⁢          p      i      D      
is minimized subject to the constraint being met on every downlink spatial channel of the conventional downlink channel.
In yet another embodiment of embodiment of the downlink power assignment step, in a conventional downlink channel, the set of powers to apply for downlink communications for the spatial channels in any conventional downlink channel are determined by setting the predicted downlink SINR in each downlink spatial channel of the conventional downlink channel to be equal to a target SINR for that downlink spatial channel. In the preferred embodiment, the target SINR is the same for all downlink spatial channels of the conventional uplink channel.
The preferred embodiments of the ongoing power control method of the present invention and of the initial power control method of the invention of the Parent Patent require an estimate of SINR of a received angle modulated signal. Another aspect of the invention is a method for determining a SINR estimate in a receiver for receiving an angle modulated signal, the method for use in a power control method for transmitting to the receiver, and for use in any other applications requiring an estimate of the quality of a received angle modulated signal. In a first implementation, the method includes estimating the mean amplitude level and the mean power level (i.e., the first and second moments of the amplitude) of the received baseband signal from measurements of the amplitude of the received signal and solving a set of simultaneous equations for the received SINR estimate.
In one particular embodiment of the first implementation, applicable for a signal modulated according to a digitally modulated scheme, the signal received in a receiver wherein the digitally modulated received signal is sampled in the receiver, the mean amplitude level and the mean power level are determined from the received baseband signal amplitude sample values, the sample values substantially at the baud points of the digital modulation scheme. In another particular embodiment of this implementation for a signal modulated according to a digital modulation scheme in a communications station having an array of antennas and a signal processor for spatial processing, wherein the digitally modulated received signal is sampled in the communication station, the mean magnitude and mean power levels are determined from the received baseband signal amplitude sample values after spatial processing, the sample values substantially at the baud points of the digital modulation scheme.
Denoting the estimate magnitude of the baseband signal as R and the estimation operation as E{ }, the set of equations is
E[R]={square root over (2"sgr"2)}ƒ(SINR),
where             f      ⁡              (        SINR        )              =                  e                  -          SINR                    ⁢                        ∑                      l            =            0                    ∞                ⁢                                            Γ              ⁡                              (                                                      3                    2                                    +                  l                                )                                      ⁢                          SINR              l                                                          Γ              ⁡                              (                                                      1                    2                                    +                  l                                )                                      ⁢                          l              !                                            ,      xe2x80x83    ⁢      and  xe2x80x83E[R2]=2"sgr"2(1+SINR).
In one version, an iterative solution is used and values of ƒ(SINR) are pre-stored in a memory. In another version, an iterative solution also is used and ƒ(SINR) is approximated by the value 1.
In a second implementation, the method for estimating the received signal quality includes estimating the mean power level and the means square of the power level (i.e., the second and fourth moments of the amplitude) of the received baseband signal from measurements of the amplitude of the received signal, and determining the SINR from these estimates. In one particular embodiment of the second implementation, applicable for a digitally modulated signal received in a receiver wherein the digitally modulated received signal is sampled in the receiver, the mean power level (the RSSI estimate) and the mean squared power level are determined from samples of the instantaneous power (i.e., the amplitude squared) substantially at the baud points. In another particular embodiment of this implementation for a digitally modulated signal in a communications station having an array of antennas and a signal processor for spatial processing, wherein the digitally modulated received signal is sampled in the communication station, the mean power level and the mean squared power level are determined by averaging post spatial processing samples of the instantaneous power (i.e., the amplitude squared) substantially at the baud points. Denoting the RSSI (the mean power level of the received signal, post spatial processing in the case of SDMA) by {overscore (R2)}, and the mean squared power by {overscore (R4)}, the SINR is determined using       SINR    =                            A                          1          -                      A                              =                        A          +                      A                                    1          -          A                      ,      xe2x80x83    ⁢                    where            ⁢              xe2x80x83            ⁢      A        =          2      -                                                  R              4                        _                                              (                                                R                  2                                _                            )                        2                          .            
In the preferred embodiment of both the first and second implementation, the SINR estimate of a received signal is determined over a single time period, preferably over a frame in the case of a PHS system. In an improved embodiment applicable to both the first and second implementation, the SINR value is determined as a running average of the SINR estimate in the current time period with SINR values determined in previous time periods.
Other aspects of the invention will be apparent to those of ordinary skill in the art from the following detailed description.